This invention relates to an image forming lens system which can rotate a final image without the necessity of an additional or special electromagnetic lens or lenses in a transmission electron microscope.
In many conventional electron microscopes, the image forming lens system (consisting of an objective lens, a projector lens and more than two intermediate lenses between the objective lens and the projector lens) forms the final electron microscope image on a fluorescent screen or a photo plate. The lenses are usually all electromagnetic types, so that the orientation (or azimuth situation) of the final image on the fluorescent screen rotates by angle .theta. about the optical axis of the image forming lens system from the orientation of the corresponding specimen. The said azimuth (or rotation) angle .theta. is given by the following equation. ##EQU1## wherein,
e is the charge of an electron;
m is the rest mass of an electron;
Vr is the accelerating voltage of the electron beam corrected for the "principle of relativity";
Bz is the magnetic field intensity along the optical axis Z of the image forming lens system;
Zs is the specimen position on the optical axis;
Ze is the final image position on the optical axis.
In the above equation, the term ##EQU2## has a constant value. And the integral value of Bz from Zs to Ze equals to the total excitation (magnetomotive force) NI (ampere turns) of the electromagnetic lenses between the specimen and the final image.
When the final image is photographed, the rectangular part at the center of the fluorescent screen is removed so that the final image is projected on the photo plate located under the fluorescent screen. So it is often desired to rotate the final image on the photo plate so as to fit that portion of the field of view being of interest into the shape of the rectangular photo plate. It is possible to vary or adjust the azimuth situation of the final image on the screen by changing the magnetomotive force NI of some lens member in the image forming lens system. By so doing, in the conventional electron microscope, however, the magnification of the final image is also varied due to changes in the focal length of the lens member in the image forming lens system.